Urban air pollution is widely recognised as being one of the greatest environmental concerns to those living or working in urban areas. The need for monitoring and obtaining highly accurate measurements of the pollution is growing constantly due to the demands of governmental institutions, environmental consciousness, and new legislation. This high accuracy of measurement is required to produce representative high resolution maps or accurate and precise positional data of pollution levels or other environmental factors in urban areas e.g. during rush hours, which can be studied to better control the urban environment, such as traffic flow management.
Unfortunately, conventional measurement systems do not provide the highly accurate detection of gases in air as required, nor do they provide a real-time map. These systems normally collect data over timescales of the order of minutes. The pollution levels are often retrieved from samples removed from the point of measurement, making correlations with pollutant sources difficult.
Other systems use fixed location stations and are usually sparsely distributed, e.g. by the roadside, that provide information data about the pollution in situ. However, it is not possible to monitor an extensive area with fixed stations and only a limited number of areas can be monitored by such as system. Furthermore, such fixed systems cannot provide a widely based real-time map. Conventional systems are also limited to being fixed near to the roadside rather than able to measure concentration values closer to the centre of road junctions or along the road itself. Conventional systems are usually set up to measure only one pollutant or other environmental factor. Furthermore, in some cases the pollutant is not measured directly, but is an extrapolated value.
Accordingly, mobile devices have been introduced to monitor the pollution levels in the atmosphere. Mobile devices here are being defined as devices capable of taking continuous measurements whilst being transported between geographic locations, e.g. those which can be mounted onto road vehicles, or carried by hand, These mobile devices are flexible and have an extensive spatial coverage. The disadvantage associated with known monitoring systems using such mobile devices is the lack of accuracy of the measurements, and limited response time, when monitoring pollution levels.
U.S. Pat. No. 6,750,467 discloses a mobile gas detector comprising a laser transmitter and signal analyzer carried on a vehicle and a laser absorption cell carried on the exterior of the vehicle.
U.S. Pat. No. 7,288,770 discloses a portable air monitoring system using UV spectroscopy capable of detecting chemicals in the open atmosphere or in a sample of air which is introduced into a chamber. The system enhances its sensitivity and accuracy by collecting a full spectrum of data points and by using multiple mirrors to increase the beam path in a closed-path length. The accuracy of these methods is good but not sufficient to fulfil the requirements that are present today, such as forming representative maps of pollution levels.
U.S. Pat. No. 6,415,646 relates to a method for the measurement of gas concentrations with a measuring head which is provided with a satellite-supported global positioning device. The position of the measuring head is determined at predetermined time intervals and positional data compared. A measured value of gas concentration corresponding to positional information can be stored in a data memory.
EP 1,942,342 describes a detection system employing trained animals. Biometric data can be collected from the animal to determine the change in body position of the animal. Thus, by analysing the body position of the animal, it can be determined whether the animal has detected a target odour. The positional information of the animal can be determined by a GPS system supplemented with an inertial navigation system. There is no means described for mapping the environmental data.
US 2006/0237657 describes real time UV spectroscopy for the quantification of gaseous toxins using open-path or closed multipass white cells. The system can be used in a portable system for UV spectroscopy capable of the detection and quantification of chemicals from either open air environments or by insertion of a sample using a sample chamber. The system can, for example, be mounted on the rooftop of a vehicle.
US 2008/0024323 describes a mobile system for monitoring travel conditions, in particular air quality. An air monitoring device can be mounted on a vehicle to collect air quality data combined with the data relating to the physical location of the vehicle. A central computer can be configured to analyse the air quality data and disseminate the information in near real time such that recipients can alter their behaviour based in part on the results.
Hideo Tai in Proceedings of the SPIE, Vol. 3746, pages 332-336 describes a methane monitoring system installed n a vehicle with a positioning system and a mobile GIS that enables real time recordal of gas leakage from buried pipelines.
EP 1,113,268 describes a method and apparatus for air quality monitoring in a predetermined geographical area wherein GPA data can be used to plot the position of the vehicle collecting the air quality samples.
In the prior art, measurements are made as the vehicle moves along, but cannot be translated into any sort of real-time map due to the lack of accurate position data required in an urban or built up environment, particularly where GPS information can be difficult to obtain.
The position calculated by a GPS receiver requires the current time, position of the satellite and the measured delay of the received signal. The position accuracy is primarily dependent on the satellite position and signal delay. To measure the delay, the receiver compares the bit sequence received from the satellite with an internally generated version. By comparing the rising and trailing edges of the bit transitions, modern electronics can measure signal offset to within about one percent of a bit time, or approximately 10 nanoseconds for the C/A code. Since GPS signals propagate at the speed of light, this represents a theoretical error of about 3 meters. Position accuracy can be improved by using the higher-chip rate signal, however, typical electronics errors are one of several accuracy-degrading effects When taken together, autonomous civilian GPS horizontal position fixes are typically accurate to about 15 meters (50 ft).
Additionally GPS signals can also be often affected by multipath issues, where the radio signals reflect off surrounding terrain; buildings, canyon walls, hard ground, etc. These delayed signals can cause inaccuracy. A variety of techniques, most notably narrow correlator spacing, have been developed to reduce multipath errors, but cannot be mitigated completely and remain a significant problem. In these environments such as an urban environment or structure-rich environment, the GPS positioning data is further corrupted very significantly or even blocked completely. This is caused by multi-pathing of GPS signals reflecting of large structures, sudden variations in positions occurring from different satellites coming into view, or limitations imposed from low-cost electronics or reception.
In the instance of multi-pathing or blockage, we can consider that the accuracy of GPS may be limited over long time periods, whereas for short-term or real-time measurements the precision of GPS determined coordinates may not be sufficient.
A speed profile obtained from the GPS data will be obtained as discrete values because the measurements of speed were performed at 1 second intervals (1 Hz). Such time series are overlaid with high frequency noise signals showing spikes in the profile. In order to remove this noise signal, a smoothing filter can be applied. However, although some smoothing filters of position may be possible, short-term accuracy may not be significantly improved.
GPS can provide height information, and additionally, GPS height information can be improved through the inclusion of additional GPS receivers mounted some distance apart. Height information is especially relevant to low level airborne sensors, such as those mounted on UAVs.
In scrutinising environmental factors, it is recognised that these factors typically vary over extremely short geographical distances, particularly in urban environments. Therefore it is necessary to collect very localised data as to environmental factors. Utilising only GPS positioning data is not sufficient to provide accurate and precise geographical positioning particularly over short timescales, but the problem is even more severe in an urban or other structure-rich environment.